Researchers and footwear brands continue to look for non-destructive ways of measuring foot movement inside a shoe, particularly in tennis, where rapid changes of direction place high demands on foot to shoe interaction. Existing methods such as cutting holes in footwear to accommodate motion capture markers compromise shoe integrity, while externally placed markers or electromagnetic tracking systems can introduce inaccuracies or interfere with natural movement. With recent advances in smartphone magnetometer technology, this project investigated whether magnetometers paired with small neodymium magnets could be used to track in-shoe inversion and eversion during tennis movements without modifying the shoe.
A magnet and IMU pair was tested alongside a Qualisys motion capture system. Initial testing focused on how magnetic field strength varied across sensor axes and distances, showing that magnet placements within 1 to 2 cm of the sensor produced the most sensitive readings. Two familiarization sessions were used to map magnetic field behaviour along the x, y, and z axes, providing a basic understanding of how the sensor responded to changes in magnet position. A lateral agility test was then performed to compare the magnetometer’s ability to detect foot movement with motion capture data. However, limitations related to marker placement and the complexity of the collected data prevented a meaningful comparison at this stage.
In the following phase of testing, a surrogate shank model was used to demonstrate changes in magnetic field strength over distances similar to those observed previously. Both static and dynamic trials were conducted, after which six-step walking tests were performed using a human participant to assess the system during simple and repeatable movement.
Across all trials, the magnetic system consistently detected changes in field strength corresponding to variations in heel to sensor distance. Discrete steps and overall movement patterns could be identified reliably. Three-dimensional plots of the magnetic field vectors provided a general representation of movement, showing that the system could capture the presence and timing of motion, but not precise positional changes. This limitation is expected, as magnetometers measure magnetic field strength rather than position, and the relationship between field strength and distance is both nonlinear and directionally ambiguous. As a result, while magnetic tracking can provide useful information about general motion and foot to shoe interaction, it cannot currently replace motion capture for accurate kinematic measurements.
Overall, magnetometers show potential as a lightweight, low-cost, and non-destructive method for identifying basic foot movement patterns inside footwear. However, higher-resolution motion tracking still requires multi-point systems such as motion capture. Future work should focus on optimizing magnet and sensor placement, further investigating individual axes, particularly the z axis, and validating the method during more controlled movements. The findings suggest that magnetic tracking may be used as a complementary tool, but not as a replacement for motion capture in performance footwear research.
